The missile inertial roll angle is an essential parameter that the missile relies on when it directs itself towards the target. Without the knowledge of its body roll angle in inertial space the missile cannot translate the inertial up, down, right, and left steering commands which guide it towards the target into adequate commands in the steering mechanisms axes. This is a common situation in the ground-assisted, mid-course phase of missiles interception processes, in the stage where the missile seeker has not yet acquired the target.
The traditional approach to the onboard inertial roll angle estimation during mid-course is to use an inertial roll-rate sensor onboard the missile and continuously integrate its output to have the inertial roll angle. It turns out however that the accuracies achievable with inertial roll-rate sensors do not quite meet the requirements for successful seeker acquisition and missile guidance. There is thus a widely recognized need for, and it would be highly advantageous to have, a method for onboard inertial roll angle estimation during mid-course, whose accuracies satisfy the mid-course guidance requirements.